Holographic elements have been extensively used in the optical frequency band for a variety of applications such as beam switching, beam shaping, data storage, etc. In a traditional holographic scheme, an interference of two beams, which are usually referred to as a reference beam and an object beam, in a photosensitive film is used to fabricate a hologram for applications in the optical frequency band. The interference pattern of the two beams can be imprinted in the photosensitive volume of the film by a post-processing, resulting in a modulation of the refractive index in a transverse plane, or in other words to an appearance of a grating-like pattern of dielectric perturbations in the film. A holographic element fabricated thereby, when illuminated by the reference beam, will reproduce the object beam due to diffraction of the former on the grating pattern. In some applications such as beam switching or beam shaping the reference beam can therefore be referred to also as an input beam, and the object beam—as an output beam.
Holograms are often divided into two categories, depending on a thickness of the grating structure and on a beam conversion efficiency defined as a ratio of the output beam power to the input beam power: thin holograms, having an essentially two-dimensional (2D) pattern of dielectric perturbations typically thinner than a wavelength of the input beam, and thick or volume holograms, which typically have thickness on the order of the wavelength or more. Generally, only a portion of the input beam power is converted to the output beam power by a hologram, with the rest being either lost due to material attenuation and spurious reflections, or transferred into a set of detrimental side beams formed by waves diffracted into lower- and/or higher-order diffraction directions. Thin holograms wherein every part of the input beam radiation within its aperture experiences a substantially single diffraction event while propagating through the film, have a limited conversion efficiency, generally not exceeding 36%, and most of the beam power is lost to the side beams propagating in other diffraction orders. Conversely, in thick volume holograms multiple consecutive diffraction events on the dielectric perturbations can theoretically increase the conversion efficiency to up to 100%, so that most of the input beam power can be transferred into the output beam. Therefore thick volume holograms have a considerable advantage over thin holograms as being potentially much more efficient in beam modifying applications.
In a microwave frequency band, sometimes also referred to as a radio-frequency band and understood herein as including the cm, mm and sub-mm wavelength range corresponding to a frequency range 10–1000 GHz, holographic elements for such applications as beam steering or beam shaping have been exploited to a much lesser degree than in optics, mostly due to difficulties of transferring the well-developed optical technologies for hologram fabrication into the microwave region.
First, traditional methods of holographic fabrication by imprinting the interference pattern of two beams in a photographic material is not easily transferable into the microwave frequency band due to a lack of efficient photo-imprinting technologies for wavelengths much higher than optical. This difficulty has been overcome in computer-generated holograms, wherein a binary or m-ary grating pattern for converting an input beam into an output beam is generated by a computer after appropriate diffraction simulations, and then transferred into either an appropriate surface profile of a dielectric film or plate for phase holograms, or into a pattern of opaque obstacles with a spatial period about or exceeding half of the beam wavelength for amplitude holograms.
Computer-generated holograms of this type for shaping and re-directing microwave beams in the mm and cm wavelength bands were described for example by J. Meltaus et al. in an article “Millimeter wave beam shaping using holograms”, IEEE Transactions on Microwave Theory and techniques, vol. 51, No.4, April 2003. U.S. Pat. No. 5,670,965 to Tuovinen et al. discloses a compact antenna test range for performing antenna and radar cross-section measurements having a transmitter for transmitting an electromagnetic wavefront and at least one radio frequency hologram of the aforedescribed type for receiving the wavefront, converting the wavefront into a plane wave, and passing on the plane wave at an oblique angle with respect to a central axis of the hologram for illuminating a piece to be tested.
However, to the best of the inventors' knowledge, no thick volume hologram for the microwave frequency band wherein the detrimental side beams are suppressed has been disclosed so far. Furthermore, heretofore low loss efficient holograms having a desirable thick three-dimensional pattern of dielectric perturbations for the microwave frequency band have been difficult to manufacture.
On the other hand, a 3D profiling of the dielectric constant at microwave frequencies can be achieved in such materials as artificial dielectrics. An artificial dielectric can be viewed as a large-scale model of an actual dielectric, obtained by arranging in a host dielectric material a large number of identical conducting or non-conducting inclusions in a regular or irregular three-dimensional lattice so that spacing between centers of the inclusions is much smaller than a wavelength of microwave field therein. Depending on the spacing of the inclusions from each other and on their size, a range of values can be realised for a dielectric permittivity of the structure, which is different from the dielectric permittivity of the host material. Similar to natural dielectrics which owe their higher than unity refractive index to an electrical polarisation of its constituent molecules in an external electric field, the inclusions in artificial dielectrics give rise to the same effect if their size and spacing are much smaller than the wavelength λ of the exerted time varying electric field, typically about or less than λ/10.
The ability of a dielectric material to be electrically polarized by an external electromagnetic field is commonly expressed in terms of relative permittivity, commonly defined as a ratio of permittivity of the material to permittivity of free space. The relative permittivity is also referred to as a dielectric constant. For purposes of this application, permittivity means relative permittivity or the dielectric constant unless otherwise indicated.
Artificial dielectrics have been used for fabrication of microwave lenses, either by appropriate shaping of the input/output surfaces of a block of an artificial dielectric as commonly done in conventional lenses, or by creating a non-homogenous medium wherein a 3D shaping of the effective dielectric constant, or the effective permittivity, is obtained by appropriately varying the size and/or spacing between the inclusions to achieve a focusing effect in a flat block of artificial dielectric.
A known method of fabricating an artificial dielectric, wherein a 3D shaping of the effective dielectric constant can be conveniently obtained, uses periodic patterns of metallic patches printed upon stacked dielectric layers. If a separation between layers and a size of metallic print is less than about a tenth of a wavelength of an incident electromagnetic wave, dielectric properties of the structure at the wavelength of the incoming wave can be characterized by an equivalent dielectric constant whose value can be changed by varying geometrical parameters of the structure such as an inter-layer spacing, size and separation of the metallic patches.
The aforedescribed layered artificial dielectric materials have been used for the manufacturing of flat inhomogeneous microwave lenses, wherein the effective dielectric constant is gradually changed on a scale of several wavelengths by a gradual variation of the inclusions' size or spatial density, so to produce a focusing effect for an incident microwave beam by means of refraction.
An object of this invention is to provide a volume hologram having a 3D lattice of inclusions arranged to form a volume dielectric grating for modifying an electromagnetic beam by diffraction thereupon.
Another object of this invention is to provide a volume hologram fabricated in an artificial dielectric comprising a plurality of dielectric sheets with 2D lattices of inclusions arranged to form a staked plurality of modulated dielectric layers for modifying a microwave beam.
Another object of this invention is to provide a method of fabrication of volume holograms for applications in the microwave frequency band using artificial dielectric technology.